Plug and play Plug-in applications are programs that extend the capabilities of your Web-browser to interactively play sound or motion video stored in downloadable files with the specific plug-in format. In earlier versions of web browsers these programs were known as "helper applications" which could only function in a separate window. A plug-in application is recognized automatically and can operate as an integrated part within the main HTML-file. As their name already suggests, plug-in applications are not supplied with your Web browser, but have to be installed beforehand. As an extra service, WWW-pages with plug-in applications often provide the link to the site from where the plug-in can be downloaded, displayed as a clickable image: By this time there are over 100 plug-in applications available. For cartographers, the most popular plug-ins are Adobe Acrobat Reader (which allows you to view documents just as they look in the print medium), Apple Quicktime (interactive animation and sound player), Macromedia Shockwave (interactive animation and sound player) and various VRML-viewers (Virtual Reality Modelling Language for interactive 3D image sequences). Plug-ins and HTML Some HTML-editors offer the possiblity to automatically insert the most common plug-in files. Manual insertion of a plug-in file requires advanced knowledge of HTML-code. Depending on the plug-in software installed by the user, the WWW-browser will either recognise the plug-in file and directly start to play, or it will display a message that no proper plug-in software is available.